The embodiments disclosed herein relate generally to micro-electro-mechanical system (MEMS) attenuators and more particularly to variable optical attenuators.
The telecommunications industry is undergoing dramatic changes with increased competition, relentless bandwidth demand, and a migration toward more data-centric network architectures. First generation point-to-point wave division multiplex systems have eased the traffic bottleneck in the backbone portion of a network. As a new cross-connect architecture moves the technology closer to the subscriber side of the network, operators are challenged to provide services at the optical layer, calling for more flexible networks that switch, attenuate and reroute wavelengths. This is placing great emphasis and demand for wavelength agile devices.
The need to provide services “just in time” by allocation of wavelengths, and further migration of the optical layer from the high-capacity backbone portion to the local loop, is driving the transformation of the network toward an all optical network in which basic network requirements will be performed in the optical layer.
The optical network is a natural evolution of point-to-point dense wavelength division multiplexing (DWDM) transport to a more dynamic, flexible, and intelligent networking architecture to improve service delivery time. The main element of the optical network is the wavelength (channel), which will be provisioned, configured, routed, attenuated and managed in the optical domain. Intelligent optical networking will be first deployed as an “opaque” network in which periodic optical-electrical conversion will be required to monitor and isolate signal impairments. Longer range, the optical network will evolve to a “transparent” optical network in which a signal is transported from its source to a destination entirely within the optical domain.
A key element of the emerging optical network is an optical add/drop multiplexer (OADM). An OADM will drop or add specific wavelength channels without affecting the through channels. Fixed OADMs can simplify the network and readily allow cost-effective DWDM migration from simple point-to-point topologies to fixed multi-point configurations. True dynamic OADM, in which reconfiguration is done in the optical domain without optical-electrical conversion, would allow dynamically reconfigurable, multi-point DWDM optical networks. This dynamically reconfigurable multi-point architecture is slated to be the next major phase in network evolution, with true OADM an enabling network element for this architecture.
On chip integration of optical switching and planar light circuits has the potential to greatly reduce the size and manufacturing costs of multi-component optical equipment such as Variable Optical Attenuators (VOAs). Current costs for Variable Optical Attenuators (VOAs) are significant, limiting their use to long-haul optical telecommunications networks. In order to extend their use into the metropolitan network the cost will need to be decreased by an order of magnitude without sacrificing performance.
One solution in particular to decreasing cost is through the integration of components, where the primary cost savings will be in packaging. A number of approaches are being pursued for optical integration using Planar Light Circuit (PLC) technologies. The majority of approaches use a silica-on-silicon platform with the VOA formed from the integration of silica Arrayed Waveguide Gratings (AWG's) for multiplexing and demultiplexing, with Thermo-Optic (TO) attenuators for performing the add/drop and pass of the demultiplexed signal. The use of a low-index contrast silica-on-silicon platform severely limits the yield of these components due to the requirement for uniform thick oxide films over large areas to form the waveguides. The use of TO attenuators limits the extensibility due to high-power requirements and thermal cross-talk.
A number of different materials and switching technologies are being explored for fabricating chip-scale photonic lightwave circuits such as AWG's for demultiplexers and multiplexers, Variable Optical Attenuators (VOA's) and Reconfigurable Optical Add-Drop Multiplexers (ROADMs). The main material platforms include silica wafers, silica-on-silicon substrates using both thin film deposition and wafer bonding techniques, polymer waveguides defined on silicon substrates, and silicon-on-insulator substrates. The main switching technologies include Mach-Zehnder interferometers based on either a thermo-optic or electro-optic effect, and MEMS mechanical waveguide switches and attenuators.
While silica waveguides have optical properties that are well matched to the optical properties of conventional single mode fibers, and thus couple well to them, they require thick cladding layers due to the low index of refraction contrast between the waveguide core and cladding materials, making them difficult to fabricate using planar processing techniques for fabrication and integration with other on-chip optical devices. The low index of refraction contrast, An, between core and cladding also requires large bending radii to limit optical loss during propagation through the photonic lightwave circuit, leading to large chip footprints and low die yields (<50%).
In addition, silica based waveguide attenuators are typically based on Mach-Zehnder interference using thermo-optic effects, that have a limited Extinction Ratio (ER) of around 25-30 dB, require significant power due to the low thermo-optic coefficient of silica, have problems with thermal cross-talk between the different optical channels and have a sinusoidal rather than a digital optical response.
What is needed is a Silicon-On-Insulator (SOI) platform for monolithically integrating optical, mechanical and electrical functions. The use of a silicon platform enables fabrication of components using the vast infrastructure and process development available for semiconductor IC manufacturing at silicon foundries. By fabricating the MEMS switches, attenuators and waveguides in the same material, single crystal silicon, there are no stress and strain issues as exist with heterogeneous materials sets such as silica-on-silicon. Fabrication in silicon also allows for integration with CMOS microelectronics for control and sensing capabilities, and for free-carrier plasma dispersion effects to enable signal leveling using integrated VOA's. The high index contrast of silicon (n=3.5) enables the ridge waveguide structures to make tight turns with minimum optical bending loss, decreasing overall chip size to centimeter dimensions.
Disclosed in embodiments herein is a micro-electro-mechanical variable optical attenuator comprising a fixed optical waveguide and a movable waveguide shuttle further comprising an optical waveguide which may be brought into substantial alignment with the fixed optical waveguide. The micro-electro-mechanical variable optical attenuator further comprises an actuator micro-incrementally misaligning the movable waveguide shuttle thereby correspondingly micro-incrementally misaligning the movable optical waveguide relative to the fixed optical waveguide. A latch is provided that will hold the movable optical waveguide as micro-incrementally misaligned, relative to the fixed optical waveguide, by the actuator, in a manner such that any optical signal passing through the fixed optical waveguide and movable waveguide is attenuated.
Also disclosed in embodiments herein is a micro-electro-mechanical optical switch and variable optical attenuator comprising a first fixed optical waveguide for selection and a second fixed optical waveguide for selection. Further provided is a movable waveguide shuttle further comprising an optical waveguide which may be brought into substantial alignment with either the first fixed optical waveguide or the second fixed optical waveguide as selected. An actuator is provided for micro-incrementally misaligning the movable waveguide shuttle thereby correspondingly micro-incrementally misaligning the movable optical waveguide relative to the selected first or second fixed optical waveguide; and a latch that will hold the movable optical waveguide as positionally micro-incrementally misaligned relative to the selected first or second fixed optical waveguide, by the actuator in a manner such that any optical signal passing through the fixed optical waveguide and movable waveguide is attenuated.
Further disclosed in embodiments herein is a micro-electro-mechanical system optical switch with integral variable optical attenuator comprising two or more fixed optical waveguides and a movable waveguide shuttle which may be brought into substantial alignment with any of the two or more fixed optical waveguides. The micro-electro-mechanical system optical switch with integral variable optical attenuator further comprises an actuator for switching the movable waveguide shuttle to a selected one of the two or more fixed optical waveguides and further capable of micro-incrementally misaligning the movable waveguide shuttle relative to the selected one of the two or more fixed optical waveguides, and a latch that will hold the movable waveguide shuttle as micro-incrementally misaligned by the actuator, relative to the selected one of the two or more fixed optical waveguides, in a manner such that any optical signal passing through is attenuated by some variably desired amount.